Magnetic resonance imaging (MRI) is a medical diagnostic imaging technique used to diagnose many types of injuries and medical conditions. An MRI system includes a main magnet for generating a main magnetic field through an examination region. The main magnet is arranged such that its geometry defines the examination region. The main magnetic field causes the magnetic moments of a small majority of the various nuclei within the body to be aligned in a parallel or anti-parallel arrangement. The aligned magnetic moments rotate around the equilibrium axis with a frequency that is characteristic for the nuclei to be imaged. This frequency is called the Larmor frequency. An external radiofrequency (RF) field applied by other hardware within the MRI system perturbs the magnetization from its equilibrium state. Upon termination of the application of the RF pulse, the magnetization relaxes to its initial state. During relaxation the time varying magnetic moment induces a detectable time varying voltage in the receive coil. The time varying voltage can be detected by the receive mode of the transmit coil itself, or by an independent receive only coil. An image processor then reconstructs an image representation from the received magnetic resonance signals for display on a human readable display.
MRI systems, such as system 10 illustrated in FIG. 1 (Prior Art), are made of many hardware components that work in conjunction with specialized software to produce the final images. Magnet 12 is the main hardware component of MRI system 10 and is responsible for producing the uniform main magnetic field, B0. Magnets used in MRI systems are very large and can have a horizontal or a vertical magnetic field. Patient table 14, commonly called the patient couch, extends into bore 16 of magnet 12, and exists to support and position patient 18 so patient 18 can lie comfortably during the imaging process. Couch 14 houses mechanical as well as electrical components that allow patient 18 and couch 14 to be moved to the center of the magnet bore 16, to a point called isocenter 20, where the most uniform and sensitive area of magnet 12 is located and imaging commonly occurs.
Within the volume defined by main magnet is at least one gradient coil 22. Gradient coil 22 produces substantially linear spatially-varying magnetic fields within the main magnetic field that are coincidental with the direction of the main magnetic field but vary along the three orthogonal directions (x, y, z) of the Cartesian coordinate system. Radiofrequency (RF) transmit coil 24 produces a perturbing RF pulse across the examination region.
One or more RF receive coils 26, commonly called imaging coils, are typically placed within the vicinity of the patient during imaging. Imaging coil 26 is comprised of a series of inductive and capacitive elements and operates by resonating and efficiently storing energy at what is known as the Larmor frequency. Coil 26, is comprised of at least one, and usually more than one element typically made of a continuous piece of copper in a loop, butterfly, figure-eight, or other continuous geometric shape. The elements are positioned at various locations throughout coil 26.
When coil 26 is used for imaging purposes each element collects information from the time varying voltage induced by the magnetic moments within the anatomy of patient 18 nearest to that element. The information collected by each element is processed through the electronics within the MRI system on individual channels of the MRI system, which keep the information from each element separate throughout the imaging process. The information from each channel of the system is then processed by reconstruction software integrated with the MRI system to combine the single images from the channels to create a complete image of the anatomy of interest.
Occasionally more than one element's information can be combined within one channel of the MRI system, allowing a broader geographic range of the anatomy of the patient to be imaged, however this poses many problems known within the art, such as isolation of cross-talk between elements that overlap in the scope of the anatomy from which elements are collecting information, and cancellation of signal between elements. MRI systems and the reconstruction software they use are limited in their abilities to combine single images from the channels, and can only process a predetermined maximum number of channels to create the complete image of the anatomy of interest.
As the design of RF coils evolve, significantly more elements are being used, and significantly more complex element geometries are being constructed, often with numerous elements overlapping the same area of the coil. The number of elements that can be used during a single imaging scan may be limited by the predetermined maximum number of channels the MRI system and reconstruction software can process. As the number of elements used within coils increases, the number of possible element combinations used in each scan also increases. Because of current system limitations, this may require a decision of which elements should be combined to produce optimal images when more elements exist within the coil than can be utilized during a single scan.
Generally, the technologist desires to use the combination of elements that will produce the scan image with the highest signal to noise ratio (SNR) and best uniformity defining the desired anatomy of interest. Individual elements must be chosen for proper coverage over the anatomy of interest, and the channel gain must be adjusted to give even coverage over the anatomy of interest and decrease hot spots, which appear bright spots in the image, or voids, which appear as dark spots in the image, in the final image. Choosing the combination of elements to meet this result is not always a clear-cut process, as the combination of too many elements, or incorrect element combinations can lead to a decrease in SNR, a decrease in uniformity because of hot spots or signal voids caused by improper coverage of the anatomy of interest, signal cancellation, or a drastic increase in reconstruction time.
Ideally, the best process for selecting the optimal element combination would include acquiring information from all elements of the at least one RF coil and determining the optimal combination during reconstruction. Unfortunately, the limited number of receiver lines and reconstruction computer power prohibits this process. Currently, manual methods of selecting coil combinations are most commonly used. The technologist usually chooses the combination desired from a predetermined list of combinations labeled with the appropriate anatomy of interest. For example, the technologist imaging with a torso coil may choose between a thoracic spine mode with the elements best suited for that area being predetermined by the designer or supplier of the coil, or an abdomen mode with the elements best suited for imaging that area being predetermined. Other commercial systems allow for the technologist to select specific elements for the desired scan. Attempts have been made in the prior art to design an automatic element selection process, but to date the results have proven to be less than satisfactory.
Currently, different methods are used for selecting the best coil elements for a specific imaging situation. Selecting the best coil elements is generally complicated by smaller coil elements. The useful imaging region of a small coil element may appear in only a portion of the image field of view, making it difficult to recognize that the small element is contributing to the final image unless the entire field of view is considered instead of a single or series of single sample reference points. Ideally, all coil elements that might usefully contribute to the final image, or even a small portion of the final image, would be selected for the optimal combination of elements. For automatically selecting the best coil elements, a method of mapping the useful region of all coil elements and comparing the useful region of each element with the location of the image slices or volume to be acquired must be used.
There are a variety of methods for accomplishing the mapping of the elements, and the comparison with the desired imaged location. One method includes producing a map of each coil element's response and determining the intersection with the region to be imaged. Another option includes collecting the signals from the region to be imaged from all coil elements and determine which of the elements contribute good signal to the region to be imaged.
U.S. Pat. No. 5,138,260 (Molyneaux) describes computer controlled switching of multiple RF coils. The patent teaches techniques that attempt to determine element position from fiducial markers. Once the coil locations are determined, positions relative to the region of interest are computed and a lookup table of coil characteristics is referenced to determine if the element should be used. At least one shortcoming associated with this patent is that fixed coil geometries that have been characterized before the patient arrives must be used for the lookup table to function properly.
U.S. Pat. No. 6,223,065 (Misic) covers automatic coil element selection in large MRI coils. The automatic selection taught in this patent is based on pre-assigned anatomy and coil selection or finding the elements closest to isocenter or closest to the anatomy of interest. At least one deficiency of this patent is that no mechanism is provided for determining which elements are best or appropriate for the given situation, and no indication is provided for how elements contributing to only a small portion of the image will be selected.
U.S. Pat. No. 6,724,923 (Ma) describes the automatic selection of multi-receiver MR data using fast pre-scan data analysis. The coil selection method of this patent is based on MR signal considerations. As taught by this patent, the signal intensity of a low resolution image acquired of each element is used as the metric to determine whether the element should be used. The method can also be run using one k-space line K-space is the frequency space domain used during the reconstruction process. The patent also teaches of using a method of determining coil suitability by the distance of the element from the anatomy of interest, as measured by signal intensity. A variety of methods to threshold or compute metrics used to select or deselect coils is also proposed in the patent.
The Ma patent (U.S. Pat. No. 6,7,24,923) offers no indication for scaling each element for equal noise output, a critical component of any MR based selection method. The low resolution images collected through the method taught in the patent may not offer any region free of signal to measure noise for equalization. Channel gain is also not considered in this patent, nor is there any clear separation of the element selection and reconstruction stages.
In the article “The NMR Phased Array” as published in Magnetic Resonance in Medicine, Volume 16, pages 192-225 in 1990, Roemer et al. discuss methods of simultaneously acquiring data from a plurality of closely positioned RF receive coils and subsequently combining the data. The article suggests overlapping adjacent coils and utilizing low input impedance preamplifiers to all coils to eliminate the problematic interactions among nearby coils. The authors found that separately receiving, storing and combining the data from each of the coils with voxel-dependant weights found by mathematical algorithms obtained the highest SNR at all points of the image. This article discusses how coils should be combined for optimal results, however does not explain any methods for automatically selecting the appropriate optimal elements from the group of multiple elements.
Therefore, as shown in the Prior Art, there is still a need for a method to automatically select appropriate elements from a group of multiple elements to form the optimal combination of elements for optimal clinical images.